1. Field of the Invention
The present invention relates to an easily calibrated weighing apparatus which combines the output pressures of a plurality of hydraulic load accepting cells and produces one output signal that is proportional to the sum of those output pressures. The output signal of the weighing apparatus is also substantially linearly related to the sum of the load accepting cell output pressures.
Weighing apparatus of this type are called "totalizers", and are commonly used in conjunction with a platform weighing device that includes a foundation, a platform, and a number of hydraulic load accepting cells interposed between the foundation and platform. Each of the load accepting cells is hydraulicly coupled to the totalizer where the fluid output pressures of all of the load accepting cells are summed and a single output signal, proportional to the sum, is generated. In this way, the totalizer compensates for variation in the output pressure from any of the individual load accepting cells which might result from unequal distribution of the load on the platform.
2. Description of the Prior Art
Load totalizer weighing apparatus of the kind briefly described above are presently known. For example, U.S. Pat. No. 2,093,141 (Sonsalla) discloses a device having a plurality of hydraulic cells, arranged vertically above one another, each comprisng an oil pressure chamber, a chamber enclosing diaphragm and a piston associated with the diaphragm. Each of the pistons is connected through a transmission plate to the piston of a single manometer which yields an output signal.
U.S. Pat. No. 2,125,483 (Blanchard) discloses a similar arrangement of vertically assembled hydraulic cells which yields a single output by combining the outputs from a number of individual load accepting cells.
Bulletin 760 entitled "Instruction Manual for EMERY Load Totalizer with Voltage Output" (1970), published by The A. H. Emery Company, the assignee of the present invention, discloses a weighing apparatus also having a vertical assembly of load totalizer cells, each of which is hydraulically coupled to a load accepting cell. Each totalizer cell comprises a cylinder and a piston. The cylinders of the respective totalizer cells are interconnected in spaced coaxial relation on a fixed main frame, and the associated pistons are interconnected in spaced coaxial relation on a floating frame. A flexible diaphragm is sealed in each cylinder and overlies the associated piston to define a fluid pressure receiving chamber, and an annular diaphragm fold interlies the free space between the side walls of each piston and cylinder. The interconnected pistons are mechanically linked to an electric load cell which yields a single electrical output signal proportional to the total of the output pressures received by the totalizer cells from the load accepting cells.
Each of the devices described above have certain drawbacks. Generally, it is difficult to calibrate the totalizer to yield the same output when each hydraulic load accepting cell is independently loaded with equivalent weight. That is, it is difficult to calibrate the weighing system comprising the platform weighing device and the totalizer to yield an equivalent output when each load accepting cell is independently loaded. Ordinarily calibration requires adjustment of the load accepting cells. Such a calibration process can be time consuming, complex and expensive since the platform weighing device may have to be dismantled to gain access to the individual load accepting cells. Because the totalizer apparatus is self-contained and is usually more accessible than the load accepting cells, it is therefore preferable to calibrate the system at the totalizer.
Moreover, hydraulic load totalizers can exhibit nonlinear output response which results from variation in the acting area of the diaphragms at different relative positions of the piston and cylinder.